Technical Abstract:
The accurate assessment of soybean rust, Phakopsora pachyrhizi, is needed by both fungicide efficacy and germplasm evaluation programs. The small size of soybean rust pustules makes them difficult to count or measure in field evaluations. An evaluation of individual leaflets should provide an accurate assessment of the difference among treatments or genotypes. To ensure repeatable evaluation, a set of images representing the range of soybean rust severity was developed with a combination of APS Assess and Adobe Photoshop (Mention of trade names or commercial products in this article is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the U.S. Department of Agriculture.)
A hand-drawn leaf outline with points the size of soybean rust pustules was scanned and imported into Adobe Photoshop, where the color and number of points were edited. These images were then evaluated in APS Assess to obtain the percent of the leaf area covered and the number of points present. Images were reworked in Photoshop to add or decrease the number of pustules to obtain the desired percent coverage and pustule number. Several cycles of evaluation and editing were needed to develop an accurate representation of each severity level. Images were developed for 30 pustules (0.1% surface coverage), 75 pustules (0.25% surface coverage), 150 pustules (0.5% surface coverage), 300 pustules (1.0% surface coverage), 750 pustules (2.5% surface coverage), 1,500 pustules (5% surface coverage) and 3,000 pustules (10% surface coverage). Images with 20, 30, 40 and 50% of the surface covered were also produced, but pustules were not counted.